1. Field of the Invention
The present invention relates to a device having one or more optical input/output and being used for selectively handling a multiplexed optical wavelength, which is used for an optical transmission system utilizing a wavelength multiplexing, and in particular to amounting structure having a small module height and also a small change of characteristics against environmental temperature variations and to a device using the aforementioned mounting structure.
2. Description of the Related Art
In an optical transmission system, the main purpose of a wavelength multiplexing was an expansion of a transmission capacity by virtue of an increased number of channels in the past. In recent years, the expected benefits are an improved added value of a transmission system and a reduction of an operation cost by an extended service menu by utilizing wavelength differences and flexible band usages.
However, sufficient benefits of a speed and a cost reduction cannot be obtained by a conventional method in which an optical signal is converted into an electric signal and handled electrically, and the electric signal is converted back into an optical signal again.
Because of this, the required is a device having a plurality of optical inputs and outputs and capable of selectively handling a wavelength multiplexed signal in the form of light.
One of such optical devices includes a wavelength selection switch which is a device capable of selectively sorting an input wavelength multiplexed signal into each wavelength.
FIG. 1 shows a basic configuration example of a conventional wavelength selection switch.
The wavelength selection switch shown in FIG. 1 has the minimum comprisal of a spectroscopic element (i.e., a diffraction grating) 13 for the purpose of applying a spectroscopy to a wavelength multiplexed optical signal, an input/output optical system (comprising an input optical system and an output optical system, that is, in FIG. 1, comprising a port 10 including a port COM and #1 through #4 ports, a collimator 11 and an expanding optical system 12), a collection optical system (i.e., a focusing lens) 14 and a movable reflector body (i.e., an MEMS (microelectro mechanical system) mirror array) 15 placed corresponding to wavelengths. Note that a wavelength selective optical receptor for receiving an optical signal of each wavelength is obtained by using a photo diode array in place of the MEMS mirror array 15 which is used for a wavelength selection switch. Meanwhile, the expanding optical system 12 expands an optical beam in only one direction as shown in FIG. 1. When the expanded light beam is collected by the collection optical system after applying a spectroscopy, the cross sectional feature of the light beam at the focal point is an elongated shape with the width being widened by the direction of the expanding optical system 12. That is, the light beam is focused on the MEMS mirror array 15 as a light spot having a shape of narrower width in the direction perpendicular to the direction “a”. This is known as a characteristic that the light can be focused narrower as the width of the light is larger.
The spectroscopic element 13 shown in FIG. 1 is an example of using a transmission grating, and the spectroscopic element 13 disperses, and outputs, wavelength components included in the input light in different directions for respective wavelengths. The movable reflector body (i.e., an MEMS mirror array) 15 is placed in a position corresponding to each wavelength along the dispersing direction of wavelengths. Changing the angle of the movable reflector body 15 along the array direction of the port enables to distribute a wavelength input from the input port COM to a discretionary output port (i.e., one of #1 through #4 ports 10, other than the COM). Note that the spectroscopic element 13 can utilize a prism.
In this structure, the state of the focused beam on the movable reflector body is the main determination factor of a transmission characteristic of the wavelength selection switch with respect to a band, an addition loss, et cetera. Therefore, a mounting position of each component needs to be stable vis-à-vis environmental variations such as ambient temperatures in order to stably obtain a good transmission characteristic.
Structures of wavelength selection switches resilient against such environmental temperature variations are disclosed in a non-patent document 1 and a patent document 1. These structures have components of a wavelength selection switch mounted in a cylindrical housing with the center of the cylinder being a reference for the components. This places the focusing point of the movable reflector body within the cylinder plane, thereby providing a structure minimally generating a displacement due to a thermal expansion. That is, the focusing position of the movable reflector body is placed closed to the center axis of the cylinder and the cylinder expands symmetrically from the center axis toward the outside, and therefore the neighborhood of the center axis is almost immune to an influence of the thermal expansion.
[Non-patent document 1] Optical MEMS 2003, page 43
[Patent document 1] U.S. Pat. No. 6,307,657
An expansion of the number of output ports is important for a wavelength selection switch in order to improve flexibility in allocating wavelengths. As shown in FIG. 1, the structure is such that an expansion of the number of ports increases the height of a module because the expansion direction of the number of ports (i.e., the direction of increasing the number of ports 10 from the ports #1 through #4 and arraying them vertically) is perpendicular to the wavelength dispersion direction. Here, the module height is defined as a module size in the direction “a” (as shown in FIG. 1) which is the arraying direction of increased output ports in the case of increasing the number of the ports.
In the meantime, the devices are mounted on a board of a standardized board size in an optical telecommunication system. Therefore, there are limitations in the module height and area size in order to mount the devices on a standard board, requiring a module height of a wavelength selection switch to be suppressed while securing a larger number of ports.
FIG. 2 shows a comprisal of a conventional wavelength selection switch.
In FIG. 2, the same comprisal as that of FIG. 1 is assigned the same reference component number. The component assigned by the same reference number has the same function as that shown in FIG. 1.
Since the module has a cylindrical structure in the conventional technique as shown by FIG. 2, a dead space is created outside the area used for the light beam (i.e., parts shown by diagonal hatching). Therefore, it is disadvantageous in terms of suppressing the module height.
In order to accordingly suppress the module height, a compact and thin body needs to be implemented by a non-cylindrical configuration creating no dead space. Due to being a non-cylindrical configuration, however, a fluctuation of beam focusing position on the movable reflector body 15 vis-à-vis the above described environmental temperature variation tends to occur.